The present invention relates to the coating of optical wave guide fibers with materials that are applied as liquids and are thereafter cured to form solid protective organic coatings on the fibers. More particularly, the invention relates to an improved apparatus and method wherein the number of inclusions in the organic coating material, particularly inclusions comprising particles, is reduced in order to improve the quality and integrity of the protective coating as cured, and thus the properties of the end product.
The application of organic coatings in liquid form to optical wave guide fibers for the purpose of protecting the glass fibers from damage is well known. A typical wave guide fiber as currently produced consists of a silica-based glass thread covered with two layers of protective acrylate coating. The glass thread acts as the wave guide and provides the vast majority of tensile strength to the fiber. The acrylate coating serves to protect the glass from damage by abrasion and/or external stresses both during the manufacturing process and in the field. In order to prevent damage during the manufacturing process, the coating is applied immediately after the glass fiber is drawn and prior to contact between the fiber and any other surface. optical fibers are being drawn at ever increasing speeds, and the apparatus for applying protective coatings must be capable of providing a high quality coating at those greater fiber draw speeds.
One problem that has been encountered in the high speed coating of glass fibers is the introduction of inclusions such as particles in the polymer coating. Inclusions adversely affect the performance of the optical wave guide fibers by creating a degradation of the mechanical properties of the coating.
For example, widely differing thermal expansion characteristics of the glass fiber and the coating become problematic in the presence of inclusions in the coating. Simple uniform stresses from tension and compression resulting from the uniform thermal expansion and contraction of the fiber and the coating do not severely affect the light-transmitting and strength characteristics of the wave guide fibers. However, uneven expansion or contraction due to inclusions in the coating causes concentrated bending stresses in both the coating and the glass fiber. Those stresses adversely affect both the light-conducting properties and the strength properties of the wave guide fiber in extreme temperature conditions.
In the case of particulate contamination of the coating layers, the more important problem is the potential for the particles to contact the glass fiber and initiate a flaw that could break instantly or grow to the point of failure on further handling. Contaminating particles are often silica-based and have sufficient hardness to easily scratch or penetrate the glass fiber. Failure analyses of wave guide fibers exhibit evidence of particles imbedded in the primary coating adjacent to the glass surface. A particle so positioned could initiate surface flaws in the glass during normal bending associated with processing steps such as spooling, or during installation of the fiber.
In a typical fiber coating process, the glass fiber is directed to a coating die assembly immediately after forming. The assembly includes a guide die, a reservoir for liquid coating material and a sizing die. The glass fiber passes through each of these components in succession. The liquid coating material adheres to the fiber and forms a coating that is later cured.
Several improvements to this process have been directed toward the reduction or elimination of bubbles in the coating. For example, a process fluid such as carbon dioxide, that is soluble in the liquid coating material, may be used to displace air entrained in a boundary layer on the surface of the optical fiber before the fiber passes through the reservoir containing the liquid coating material. The process fluid travels with the fiber into the liquid coating material and dissolves into the material rather than forming bubbles as would air.
Such a process is described in U.S. Pat. No. 4,792,347, assigned to the same assignee as the present application, and which is hereby incorporated by reference in its entirety herein. In that system, a conditioning unit is positioned around the incoming fiber for providing a countercurrent gas flow for displacing the entrained air on the optical fiber. An inner cylindrical sleeve of the conditioning unit has multiple gas flow orifices that direct the countercurrent gas flow onto the fiber.
A process fluid has also been directed through slots in a cooling means to form flows that are directed toward the fiber, as disclosed in U.S. patent application Ser. No. 08/409,231, which is assigned to the same assignee as the present application. In that device, helium is used to cool the fiber and to displace or strip air from the fiber at very high draw speeds. The excess helium and any entrained air stripped from the fiber is exhausted through a port in a direction away from the fiber.
While the displacement of entrained air with soluble process fluid has produced acceptable results in the reduction of bubbles in the fiber coating, other improvements in the process have also been attempted. In U.S. Pat. No. 5,127,361, the geometry's of the guide die and the sizing die are adjusted in order to reduce the number of bubbles formed in the coating at high draw speeds. In that apparatus, the size of the gap between the guide die and the sizing die is adjusted in combination with a taper of the hole in the sizing die to improve the coating process.
Another existing die assembly introduces a process fluid into a cylindrical chamber surrounding the portion of the fiber traveling into the guide die. The chamber vents to atmosphere at an open end opposite the guide die, and has a diameter of approximately 13 mm (0.500 inches). The process fluid is introduced through one or more passageways leading to the chamber near the guide die. The process fluid is allowed to flow out the open end of the chamber in a direction opposite the direction of travel of the fiber.
While the foregoing systems have been somewhat effective in reducing defects in liquid applied coatings, further improvement would be desirable. Current processes used for coating wave guide fibers do not effectively eliminate particulate contamination of the liquid coatings.